I figure there’s not many Corvette DIYs who test coolant. Isn’t it easier to pour in some new antifreeze when you change coolant every few years and hope for the best? Easier? Heck, yeah. Smarter? No way. You’re better-off knowing how much freeze protection you have.

I have two Corvettes with different mixes of coolant in their cooling systems. One is protected down to 25° and other uses straight water. I’ve also got an old Chevy Malibu and a newer Camaro, both of which are, also, protected to 25°F. I run less, or sometimes no antifreeze, along with Red Line Water Wetter which contains a corrosion inhibiter, because 1) I live where it doesn’t get that cold and 2) coolant mixes with low or no antifreeze cool better. Ok. There’s more to the reduced antifreeze story than that, but the details are best saved for a different Corvette Action Center article.

This is a cheap hydrometer I purchased from an autoparts store. It cost all of a couple of bucks and I got what I paid for. It's readings are quite inaccurate. Not only that they are inconsistent, varying quite a bit from test-to-test. Save your couple of bucks for a cold beer. Then you can sit around the bar guessing at your antifreeze. The "beer-driven guess" method will be more accurate.

These cars get coolant changes every 18-24 months. Plus, I sometimes change antifreeze concentrations during various test programs or for trips to places having different climates. Keeping track of different coolant mixes sometimes got confusing and I was tired of guessing. I needed some way to immediately, quickly and accurately test antifreeze.

I tried two different coolant hydrometers. I quickly discovered they: 1) differed quite a bit in readings and 2) didn’t accurately test samples of coolant I knew contained specific percentages of antifreeze.

Later, I learned hydrometers are notoriously inaccurate for measuring the freezing point of aqueous engine coolants. The best units might have accuracy of +/- 8°F and their readings are temperature dependent. Sampling technique is critical to hydrometer use. Air bubbles cause inaccurate readings and float must be kept away from the wall of the hydrometer. No hydrometer can properly measure the propylene glycol antifreezes which some people use because, up to 70% concentration, propylene glycol’s specific gravity increases, but above 70%, its specific gravity decreases. A 100% solution reads identical to a 40% solution, thus a hydrometer is unreliable. In short, hydrometers are a pain in the butt to use for coolant testing and they are inaccurate, to boot.

One day I was leafing through a GM, Corvette Service Manual and ran across the Kent-Moore Duo-Chek Coolant and Battery Tester (PN J-23688). A service tool for GM dealers, this device is made for Kent-Moore by Leica Optical Products and is properly termed a “refractometer” because it relies on a coolant sample’s refraction of light to accurately determine antifreeze concentration.

While it's a bit expensive, perhaps at $169, the Kent-Moore Coolant tester is one way to quickly and accurately determine how your cooling system is protected against freezing. I own five different cars. Their cooling system mixes differ a bit and I'm often changing stuff around for testing or experimentation. For me, the Kent-Moore tester simplified my cooling system work.

Kent-Moore’s Coolant Tester is simple to use. Draw a sample from the cooling system, either from the radiator, the expansion tank or the high-fill bottle, using the tool’s “dropper bulb”. Put a drop of it on the device’s measuring prism, close the sample cover, look into the eyepiece and point the other end at any bright light. You’ll see easty-to-understand display of your antifreeze percentage and the coolant’s freeze temperature that is dead-nuts accurate...to within 1°F. Finally, my problems with quick, reliable testing of the freeze protection in my various cooling systems were solved. I’m very satisfied by the Kent-Moore Coolant tester’s accuracy and convenience of use. It’s a welcome addition to my tool box.

Using the Duo-Chek is very simple. Simply dip into the cooling system (here, we are using the high-fill bottle on a C4) and extract a sample with the dipper bulb.

Then, deposit the same on the Coolant Tester prism and close the sample door.

Point the Tester at a light source, look through the eyepiece and you'll see a temperature scale. Part of it will be in red and part of it will be white. The division line between the two corresponds to the freeze protection in your cooling system. Immediate. Simple. Accurate.

The Kent-Moore Coolant Tester comes with sampling devices for both cooling systems and batteries having removable cell caps. It works with both ethylene glycol (including GM/Texaco “DEX-COOL”) and propylene glycol coolants along with battery acid. It reads antifreeze protection down to -60°F and can be recalibrated by the user if necessary. It also reads a battery’s state of charge. It has a two position eyepiece to accommodate persons who wear glasses.